1. Field of the Invention The present invention relates to an optical pickup apparatus for executing at least one of processes of recording and reproducing information onto/on a recording medium by irradiating the recording medium with light.
2. Description of the Related Art
Recording mediums usable in an information recording/reproducing apparatus include a compact disk (abbreviated as CD), a digital versatile disc (abbreviated as DVD), and a Blu-ray disc (registered trademark). A recording/reproducing process of the CD is executed using light in an infrared wavelength region around 780 nm. A recording/reproducing process of the DVD is executed using light of which wavelength is shorter than that of the light used for the recording/reproducing process of the CD; to be specific, red light in a wavelength region around 650 nm. A recording/reproducing process of the Blu-ray disc is executed using blue-violet light in a wavelength region around 405 nm. As a widely-used light-assisted recording medium(hereinafter may be referred to as “an optical recording medium”), a plurality of recording layers are formed in one recording medium into a multilayer structure, resulting in increase in recording capacity.
Regarding reproduction and recording of information on/onto the optical recording medium, a pit indented in a recording layer of an optical recording medium for only reproduction and a track consisting of a land and a groove formed in a recording layer of an optical recording medium for recoding are irradiated with laser light, and reflected light of the laser light is detected by a light-receiving element to execute reproduction or recording of the information and moreover, a servo signal is detected to conduct a servo control.
However, in the optical recording medium having a multilayer structure is generated not only light reflected-by a light-condensed recording layer, but also light reflected by a recording layer other than the light-condensed recording layer. The light reflected by a recording layer other than the light-condensed recording layer is referred to as stray light. Such light, as well as the light reflected by the light-condensed recording layer, enters a hologram element serving as light-splitting unit for guiding the reflected light to the light-receiving element, where the light is then diffracted to further enter the light-receiving element where the stray light functions as noise, resulting in superposition of the noise onto the servo control signal. Consequently, there arises a problem that the servo control cannot be stably conducted.
An optical pickup apparatus has been proposed that solves the problem caused by the stray light generated at the time of recording and reproducing information onto/on an optical recording medium having a multilayer structure (for example, see Japanese Unexamined Patent Publication JP-A 2004-303296 and Japanese Unexamined Patent Publication JP-A 2004-288227).
FIG. 15 is a schematic view showing a configuration of a related-art optical pickup apparatus 1. FIG. 16 is a schematic view showing configurations of a hologram element 4 and light-detecting unit 7 provided in the optical pickup apparatus 1 shown in FIG. 15. In the following descriptions regarding the optical pickup apparatus 1, a tangential direction to a track formed on a recording medium 8 is defined as an X-axis direction, and a radial direction of the recording medium 8 is defined as a Y-axis direction. Note that the radial direction of the recording medium 8 represents a direction along one radial line connecting a point at an intersection between an optical axis of light emitted by the optical pickup apparatus 1 and a recording surface of the recording medium 8, with a center of the recording medium 8. Further, the radial direction is perpendicular to the tangential direction in the recording surface of the recording medium 8. Further, a Z-axis direction represents a direction which is perpendicular to the X-axis direction and Y-axis direction and which is parallel to an optical axis 14 of light emitted by a light source 2.
The optical pickup apparatus 1 comprises a light source 2 for emitting light, a diffraction grating 3, a hologram element 4, a collimation lens 5, an objective lens 6 serving as light-condensing unit, and the light-detecting unit 7 having a plurality of light-receiving elements. The optical pickup apparatus executes at least one of processes of recording information onto the recording medium 8 and reproducing information recorded on the recording medium 8.
For the light source 2, a semiconductor laser element is used, for example. Light emitted by the light source 2 is split by the diffraction grating 3 into at least three beams; namely, a main beam 11, a first sub beam 12, and a second sub beam 13. The diffraction grating 3 has a structure in which concavities and convexities are regularly formed to diffract the emitted light so as to generate a plurality of beams. The main beam 11 is a main luminous flux for taking out information recorded on the recording medium 8. The sub beams 12 and 13 are sub luminous fluxes which are used for performing control on a light-condensing position of the main beam 11. The light split by the diffraction grating 3 proceeds through the hologram element 4 and then proceeds through the collimation lens 5 by which the light is made to substantially parallel light, thereafter being guided to the objective lens 6.
The objective lens 6 condenses the light emitted by the light source 2 onto a recording layer which contains information recorded on the recording medium 8. The objective lens 6 is supported by an actuator (not shown) so as to be displaceable in both of directions which are parallel to and perpendicular to the optical axis 14 of the emitted light. Displacement of the objective lens 6 in the direction which are parallel to and perpendicular to the optical axis 14 of the emitted light, causes change of the light-condensing position of the emitted light onto the recording medium 8.
The main beam 11 and the first and second sub beams 12 and 13 are condensed by the objective lens 6 onto the recording medium 8. The main beam 11 and first and second sub beams 12 and 13 which have been reflected by the recording medium 8 proceed through the objective lens 6 and then proceed through the collimation lens 5, thereafter being guided to the hologram element 4.
The hologram element 4 serving as light-splitting unit is disposed between the light-detecting unit 7 and the objective lens 6. In the hologram element 4 are formed a hologram pattern having a plurality of divisions for splitting the light reflected by the recording medium 8 into a plurality of light beams.
The hologram pattern of the hologram element 4 provided in the optical pickup apparatus 1 is formed so that an outline of the hologram pattern when seen from one side in the Z-axis direction has a substantially-circular shape. The hologram pattern of the hologram element 4 is firstly divided into a first division 21 and a remaining part by a first dividing line 16 which is substantially parallel to the Y-axis direction that is a radial direction of the recording medium 8 placed in an information recording/reproducing apparatus so as to be in a recording or reproducing state. The first dividing line 16 contains in a center part thereof a semicircular curved section 16a. The remaining part is further divided into a second division 22 and a third division 23 by a second dividing line 17 which is parallel to the X-axis line that is a tangential direction to a track formed on the recording medium 8.
The first division 21 is formed so as to have a semicircular protruded region 24 which protrudes toward the second and third divisions 22 and 23 beyond a virtual dividing line 18 connecting both ends of the first dividing line 16 in the semicircular curved section 16a of the first dividing line 16. Accordingly, the second and third divisions 22 and 23 are each formed into a shape of substantial quarter of annulus ring.
The light-detecting unit 7 has eight light-receiving elements 7a, 7b, 7c, 7d, 7e, 7f, 7g, and 7h. Each of the light-receiving elements 7a to 7h has a substantially-rectangular shape when seen from one side in the Z-axis direction, and is disposed so that a longitudinal direction of the substantially-rectangular shape-is parallel to the Y-axis direction. In the light-detecting unit 7, the light-receiving elements 7e, 7c, 7f, 7a, 7b, 7g, 7d, and 7h are disposed in the X-axis direction in this order. Each of the light-receiving elements 7a to 7d receives light of the main beam 11 reflected by the recording medium 8. Each of the light-receiving elements 7e to 7h receives light of the sub beams 12 and 13 reflected by the recording medium 8.
When the light of the main beam 11 reflected by the recording medium 8 enters the hologram element 4, light diffracted by the first division 21 falls onto a boundary between the light-receiving element 7a and the light-receiving element 7b, and light diffracted by the second division 22 falls onto the light-receiving element 7d, and light diffracted by the third division 23 falls onto the light-receiving element 7c. When the light of the sub beams 12 and 13 reflected by the recording medium 8 enters the hologram element 4, light diffracted by the first division 21 does not fall onto the boundary between the light-receiving element 7a and the light-receiving element 7b, but falls onto positions on both sides of the light-receiving element 7a and the light-receiving 7b, where no light-receiving elements exist, and light diffracted by the second division 22 falls onto the light-receiving element 7g and the light-receiving element 7h, and light diffracted by the third division 23 falls onto the light-receiving element 7e and the light-receiving element 7f. 
A focus error signal (abbreviated as FES) acting as a servo control signal for focusing is generated from a signal detected by the light-receiving element 7a and light-receiving element 7b. A tracking error signal (abbreviated as TES) acting as a servo control signal for tracking is generated from a signal detected by the light-receiving elements 7c to 7h. 
The generation of the FES and TES as mentioned above represents generation of signals through the light reflected by a light-condensed recording layer on which light is being condensed by the objective lens 6. In a case where the recording medium 8 has a multilayer structure, there is also generated light reflected by a recording layer other than the light-condensed recording layer.
FIG. 17 is a view of assistance in explaining outlines of processes related to transmission and reflection of light on the recording medium 8 having two recording layers. In FIG. 17, in order to avoid complications so as to facilitate understanding, only transmission and reflection of the main beam 11 are shown and further, FIG. 17 is shown on the assumption that a refractive index of the recording layer is identical to that of air. In the recording medium 8 having two recording layers, a recording layer positioned close to the objective lens 6 is referred to as a first recording layer 25a while a recording layer positioned away from the objective lens 6 is referred to as a second recording layer 25b. A distance between the first recording layer 25a and the second recording layer 25b is referred to as td (hereinafter may be referred to “an interlayer distance”).
The main beam 11 exiting from the objective lens 6 is reflected by the first recording layer 25a which is a light-condensed recording layer. The resultant direct reflection; namely zero-order diffracted light 11a proceeds through the objective lens 6 and collimation lens 5 again and is further diffracted by the hologram element 4 to fall onto each of the light-receiving elements 7a to 7d of the light-detecting unit 7. This is the same as in the above-described case.
However, the main beam 11 contains a component which is transmitted by the first recording layer 25a. Such remaining light transmitted by the first recording layer 25a enters the second recording layer 25b, a track on which reflects the light to generate zero-order diffracted light 11b and + first-order diffracted light 11c. In the following description, a symbol “+” of the + first-order diffracted light 11c will be omitted.
The zero-order diffracted light 11b generated by the second recording layer 25b is reflected so as to be directed toward the objective lens 6 as if a light-condensed point is positioned away from the second recording layer 25b by the interlayer distance td. The zero-order diffracted light 11b is reflected so as to be directed toward the objective lens 6 at the same angle as an angle of incidence of main beam 11 entering the second recording layer 25b. The zero-order diffracted light 11b is transmitted by the first recording layer 25a and proceeds through the objective lens 6 and collimation lens 5 again to be more condensed than the light reflected by the first recording layer 25a, thereafter entering the hologram element 4.
FIG. 18 is a view of assistance in explaining a state where the light diffracted by the second recording layer 25b enters the hologram element 4 and the light-detecting unit 7. The zero-order diffracted light 11b diffracted by the second recording layer 25b enters the protruded region 24 of the first division 21 in the hologram element 4, and forms a zero-order diffracted light spot 26. In the hologram element 4 of the optical pickup apparatus 1, a diameter of the semicircular curved section 16a separating the protruded region 24 from the second and third divisions 22 and 23 is determined so that the resulting zero-order diffracted light-spot 26 does not extend to the second and third divisions 22 and 23 by which light for generating TES is diffracted.
The zero-order diffracted light 11b reflected by the second recording layer 25b is all made to enter the protruded region 24 as described above and by so doing, the zero-order diffracted light 11b is no longer diffracted toward the light-receiving elements 7c to 7h for generation of TES. Consequently, the zero-order diffracted light 11b is never received as stray light and it is therefore possible to obtain TES by light not including stray light generated through the reflection on the second recording layer 25b which is different from the first recording layer 25a serving as a light-condensed recording layer.
The zero-order diffracted light 11b incident on the protruded region 24 included in the first division 21 is diffracted by the first division 21 so as to fall onto the light-receiving elements 7a and 7b for generating FES as stray light. The fallen light on the light-receiving elements 7a and 7b forms an extremely blurred fallen spot 27, and an intensity distribution of light incident as stray light is spread out, resulting in a sufficiently small amount of signal with respect to the light reflected by the first recording layer 25a. Moreover, FES is obtained by a differential between the light-receiving element 7a and the light-receiving element 7b and therefore, in a case where the light enters both of the light-receiving elements 7a and 7b, the light on the light-receiving element 7a and the light on the light-receiving element 7b are cancelled out, thereby suppressing noise caused by those light.
As shown in FIG. 17, there exists not only the zero-order diffracted light 11b, but also higher-order diffracted light; namely, the first-order diffracted light 11c as light which is transmitted by the first recording layer 25a and then reflected and diffracted by the second recording layer 25b. In this context, the higher-order diffracted light indicates the first-order or higher-order diffracted light.
The first-order diffracted light 11c having a higher diffraction order than that of the zero-order diffracted light 11b, as shown in FIG. 18, enters the hologram element 4 so as to form thereon two light spots; namely a first light spot 28 and a second light spot 29 between which the zero-order diffracted light spot 26 lies and which are arranged in the Y-axis direction. The first-order diffracted light 11c is incident on the first dividing line 16 around which the first light spot 28 expands in the first division 21 and second division 22 on the hologram element 4 while the first-order diffracted light 11c is incident on the first dividing line 16 around which the second light spot 29 expands in the first division 21 and third division 23 on the hologram element 4. Since the first and second light spots 28 and 29 are formed respectively in the second division 22 and third division 23 where light for generating TES is diffracted, the light incident on each of the second and third divisions 22 and 23 is diffracted and then guided to the light-receiving elements 7c to 7h for detecting TES.
With the first-order diffracted light diffracted in the second division 22 are irradiated the light-receiving elements 7f, 7d, and 7h over which a widely expanded first fallen spots 30 is formed. With the first-order diffracted light diffracted in the third division 23 are irradiated the light-receiving elements 7e, 7c, and 7g over which a widely expanded second fallen spot 31 is formed. The light-receiving elements 7f, 7d, and 7h and the light-receiving elements 7e, 7c, and 7g are provided originally for the purpose of receiving the sub beams 12 and 13. Since light intensity of the sub beams 12 and 13 is low; that is, approximately one tenth of the light intensity of the main beam 11, a signal amplification level for a detected output through the light-receiving element for receiving the sub beams 12 and 13 is set to be larger than a signal amplification level for a detected output through the light-receiving element for receiving the main beam 11.
Consequently, detected signals from the first and second fallen spots 30 and 31 formed of light acting as stray light are also largely amplified, with the result that the stray light; namely, the first-order diffracted light 11c diffracted by the second recording layer 25b has a large impact on the light reflected by the first recording layer 25a, and the stray light generates unnecessary noise components and offset components. As a result, there arises a problem that track servo characteristics are deteriorated.
That is to say, the optical pickup apparatus 1 proposed in JP-A 2004-303296 has a problem that it is not possible to eliminate influences of the higher-order diffracted light on a recording layer other than the light-condensed recording layer, although the optical pickup apparatus 1 can eliminate influences of the zero-order diffracted light that is light simply reflected by a recording layer other than the light-condensed recording layer.